Dr. T logo BIO 405/505 Plant Physiology
Osmosis and Water Potential

Importance of water to plant function

•  Turgor pressure

•  Turgor can develop because the cell wall provides resistance against the pressure that builds up within the cell due to osmosis

•  Most cells must have a positive pressure to function.

•  Typical pressure is 5-30 times atmospheric pressure.

•  Turgor provides support (hydrostatic "skeleton").

•  When turgor is lost, softer organs wilt.

•  Growth

•  Cells expand when turgor is sufficient to deform cell wall.

•  Growth is a function of turgor and cell wall "stiffness."

•  Stomatal function

•  Opening and closing of stomata is regulated by uptake or loss of water by guard cells. This is an osmotic phenomenon.

•  Solvent for transport

•  Solutes traveling through xylem and phloem are dissolved in water.

•  Heat exchange

•  High specific heat provides a temperature buffering effect.

•  High heat of vaporization provides evaporative cooling.

•  Reactant for metabolic reactions

•  Many cellular reactions require or produce water.

•  Water is a reactant for photosynthesis.

Properties of water

•  Hydrogen bonding

•  Water is a polar molecular

Water-a polar molecule
Introduction to Plant Phyiology, Copyright John Wiley & Sons

•  Its properties are mostly due to hydrogen bonding.

•  Liquid at room temperature

•  This is significant because simlar-sized molecules are gases, e.g. ammonia, methane, carbon dioxide.

•  High specific heat

•  Specific heat is the amount of energy it takes to raise the temperature of a substance.

•  Because water's specific heat is high, it acts as a temperature buffer, a plant changes temperature (both up and down) more slowly than the air around it.

•  High heat of vaporization

•  Heat of vaporization is the amount of energy it takes to convert a substance from liquid to gas.

•  Because water's heat of vaporization is high, a lot of energy is absorbed as water evaporates from the plant.

•  This creates a cooling effect, which can be important for plants living is very hot climates.

•  High cohesive and adhesive forces

•  Important for xylem function because water molecules must "stick together" to keep xylem water columns intact.

•  High surface tension

•  At an air-water interface, water molecules are attracted more strongly to each other than to the air.

•  This creates a kind of "skin" on the water surface

Surface tension
Water droplet illustrating the surface tension effect at the air-water interface.
Introduction to Plant Phyiology, Copyright John Wiley & Sons

•  Keeps water from entering stomata and saturating the air spaces needed for gas exchange.

•  Very good solvent

•  Water can dissolve a wide variety of polar and ionic compounds. This makes an ideal medium to build cells out of.

Translocation of water

•  Water moves from one place to another in response to a gradient of energy, a difference of energy content between two regions.

•  Water movement is usually passive-going down the gradient from high energy content to lower energy content.

•  There are several components to energy content.

•  In most cases, the most important components are pressure and the effect of solutes.

•  Bulk flow

•  The substance moves from high pressure to low pressure.

•  Molecules move en masse, as a stream.

•  The high and low pressure regions must be connected by a pathway that permits mass flow.

•  Examples

•  Flow of water out of a spigot

•  Flow of sap through the phloem

•  Diffusion

•  The substance moves from high to low chemical potential.

•  In simple systems (same temperature and pressure throughout), high to low concentration explains diffusion.

•  In plant cells we will define water potential (chemical potential of water)

•  Why is water concentration alone not enough to explain diffusion of water?

•  Changes in temperature have little effect on water concentration.

•  Changes in pressure have little effect on water concentration because water is virtually incompressible.

•  Changes in solute concentration have little effect on water concentration.

•  BUT temperature, pressure, and solutes have large effects on diffusion of water.


•  Osmosis is the diffusion of water across a selectively permeable membrane.

•  Osmosis occurs when there is a water potential gradient between regions separated by a membrane.

Osmosis occurs in response to a water potential gradient
Introduction to Plant Phyiology, Copyright John Wiley & Sons

•  The diagram shows a system of two chambers separated by a membrane that is only permeable to water.

•  The right side contains pure water (no solutes)

•  The left side contains a solution (water + solutes)

•  Because pure water has a higher water potential than a solution, there is a gradient, and osmosis takes place.

•  Water moves from the region of higher water potential to the region of lower water potential.

•  Osmosis is a passive process (does not require an input of energy)

•  A system having a gradient of water potential contains potential energy. It is this stored energy that powers osmosis.

•  So when osmosis occurs, energy is released. Osmosis is a downhill process.

•  The energy released by osmosis pushes the level of liquid upward in the tube on the left side. The height of the fluid in that tube can be used as a way to measure the force of osmosis. (This is how an osmometer works.)

•  When water enters a plant cell by osmosis, pressure builds up as the membrane pushes against the cell wall. This pressure (turgor pressure) must be considered as part of the water potential of a cell.

Water potential and its components

•  Water potential (Ψ)

•  Water potential, in simplified terms, is a measure of the availability of water.

•  In more formal terms, water potential is the chemical potential of water in a particular environment.

•  Water will move from one "container" (a cell, for example) to another if the water potential is not equal in each container.

•  Water moves "down" the water potential gradient, form high water potential to low water potential.

•  So if the water potential outside a cell is greater than inside, water will enter the cell by osmosis.

•  From the standpoint of plant physiology, the important components of the value of water potential are:

•  Solute potential (Ψs) - due to dissolved solutes.

•  Pressure potential (Ψp) - due to turgor pressure.

•  Matric potential (Ψm) - important when matric material is present.

•  Solute potential (Ψs)

•  This is the part of a system's water potential that is due to the presence of dissolved solutes.

•  It is sometimes called osmotic potential.

•  For pure water, its value is zero.

•  Its value is always negative for any solution and becomes lower (more negative) with increased solute concentration.

•  Pressure potential (Ψp)

•  This is the part of a system's water potential that is due to pressure.

•  Defined as zero at atmospheric pressure

•  Can be positive, zero, or negative (tension).

•  Has a positive value in most living plant cells (turgor pressure).

•  Is negative in xylem vessels and tracheids.

•  Matric potential (Ψm)

•  This is the part of a system's water potential that is due to the presence of "matrix" material-very finely divided insoluble material with high surface area, e.g. clay.

•  Matrix material is usually near zero except for soil when near dryness and in cell walls when not saturated with water.

•  We will ignore matric potential most of the time.

Units for water potential

•  Osmotic pressure can be measured with an osmometer.

Osmotic pressure
Osmometer illustrating the development of osmotic pressure
Introduction to Plant Phyiology, Copyright John Wiley & Sons

•  Common pressure units used are:

•  Bar - a standard metric unit of pressure = 1 dyne/cm2

•  Atm (atmosphere) 1 atm is typical air pressure (approx 1 bar)

•  MPa (megapascals) 10 bars = 1 MPa

•  MPa are is the preferred unit in SI units.

•  Possible values of water potential components

Calculating solute potential

•  Ψs = -miRT (van't Hoff equation)

•  m = molality (mol/kg) (close to molarity at low concentrations)

•  i = ionization constant

•  R = gas constant = 8.31 x 10-3 kg•MPa / mol•°K

•  T = absolute temperature (°K)

•  Example, for a 0.5 molal sucrose solution at 25 C:

•  Ψs = -(0.5 mol/kg)(1)(8.31 x 10-3 kg•MPa / mol•°K)(273+25 K) = -1.24 MPa (= -12.4 bars)


•  Normal living cells have positive turgor pressure (Ψp > 0).

•  The plasma membrane is pressed against the cell wall with a pressure greater than atmospheric pressure.

•  If cells are in an environment of very low water potential, they may lose enough water so that the turgor pressure falls to zero.

•  At this point, the membrane is still against the wall, but not pressing against it.

•  If the cell continues to lose water, the volume of the cell decreases and the membrane shrinks away from the cell wall.

•  The cells are now said to be plasmolyzed.

Normal (left) and plasmolyzed (right) cells of Elodea.
Micrographs by John Tiftickjian

•  In a plasmolyzed cell, Ψp = 0, so Ψ = Ψs.

•  Incipient plasmolysis

•  Consider a turgid cell that is placed in a solution where: Ψ(outside) = Ψs (inside)

•  Initially, the cell has a positive turgor pressure, so its water potential is greater than for the solution outside.

•  The cell loses water, and its pressure potential decreases.

•  Because the solute potential of the solution is equal to the solute potential of the cell, once the cell's pressure potential is zero, the cell will have just reached equilibrium.

•  Both pressure potential and solute potential are now equal both inside and outside the cell, so osmosis stops.

•  At this point, turgor pressure is zero (Ψp = 0), but the membrane does not contract from the wall.

•  This point is defined as incipient plasmolysis.

Water potential problems (see handout)

•  Basic points to remember

•  Water always moves from higher Ψ to lower Ψ - down the water potential gradient.

•  For any location, Ψ = Ψs + Ψp + Ψm

•  We will ignore matric potential, so Ψ = Ψs + Ψp

•  When cell is at equilibrium with its surroundings, Ψinside = Ψoutside .

•  At equilibrium, osmosis does not occur - there is no net movement of water into or out of the cell.

•  When a cell is plasmolyzed, Ψp = 0.

•  At "incipient plasmolysis," there is zero turgor, but the plasma membrane has not yet withdrawn from the wall.

•  Practice problems like the ones on the handout.

Water potential and cell volume changes

•  A cell is not a perfect osmometer.

•  The cell wall is flexible - it "gives" as cell takes up water.

•  This means the cell's volume changes.

•  The change in volume affects the concentration of solutes.

•  So Ψs as well as Ψp changes as water enters or leaves a cell.

•  This relationship can be summarized with a Höfler plot

•  The graph shows the relationship between cell volume and values of solute, pressure, and water potentials.

•  As cell takes up water and volume increases...

•  Solute potential rises because cytoplasm is diluted.

•  Pressure potential rises once membrane pushes against cell wall.

•  Water potential rises as the sum of solute potential and pressure potential.

Typical potential values within the plant

•  For the plant to stay healthy, cells must maintain a proper osmotic balance and water must move from soil, through the plant, to the atmosphere.

•  Water must move through a plant down the water potential gradient.

Water potential gradient
Water flows down a water potential gradient
Introduction to Plant Phyiology, Copyright John Wiley & Sons

•  There must be a water potential gradient from root to leaf.

•  Turgor must be high enough.

•  Representative values for "typical" plant (MPa)

Measuring water potential components

•  Water potential

•  Measuring water potential is done by placing samples of plant tissue in an a series of environments of known water potential to determining the equilibrium point.

•  Tissue volume method

•  Prepare a series of solutions of know solute potential

•  Cut sample of tissue and weigh

•  Soak sample in solutions, allow time for water exchange

•  Re-weigh samples, calculate % change in weight

•  Determine the potential that the tissue would have initially been in equilibrium with.

•  The equilibrium solution then has the same water potential as the tissue did.

Measuring water potential
Measuring water potential by the volume (weight) change method

•  Assumes that the only thing that causes weight change is water uptake or loss.

•  This is not completely true. The cell wall and intercellular spaces may absorb some of the solution which increases the weight of the tissue even though water did not enter the cell.

•  Chardakov method

•  Similar to the tissue volume method, but detects the change in the soaking solution rather than in the tissue.

•  The density of the solution increases or decreases if water moves into or out of the solution from the tissue.

•  Vapor pressure method

•  Uses a device called a thermocouple psychrometer

•  Tissue is placed in a small chamber and allowed to equilibrate with the air in the chamber

•  The relative humidity (RH) in the chamber is measured with the psychrometer.

•  Water potential of the air space is calculated.

•  Water potential of the tissue is the same value because the tissue had come to equilibrium.

•  This is usually the preferred method, but must be done very precisely because water potential changes greatly with small changes in RH.

•  Solute potential

•  Freezing point depression

•  The freezing point of water decrease as solute concentration increases.

•  Grind up tissue, extract the cell sap, measure its freezing point.

•  solute potential can be calculated from the difference in freezing point between the cell sap and pure water.

•  Only an estimate because the cytoplasm of cells in not an "ideal" solution.

•  Incipient plasmolysis method

•  Can be used with some plant where plasmolysis can easily be observed with a microscope.

•  You try to find the solution which will just cause the cells to start to plasmolyze.

•  At this point, the solute potential of the cells is equal to the solute potential of the solution, since there is now zero turgor pressure.

•  Difficult to use in practice, because plasmolysis is difficult to observe accurately in most plants.

•  It does work fairly well for aquatic plants with thin leaves.

•  Pressure potential

•  Can be measured directly only in very large cells

•  Usually just calculated from water potential and solute potential

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